Hot melt adhesives are thermoplastic materials that can be heated to a melt, and then applied to various substrates. A bond is formed upon cooling and resolidification of the melt. These hot melt adhesives have the drawback of often becoming brittle below the glass transition temperature. Historically, ethylene-based semi-crystalline polymers like polyethylene and ethylene vinyl acetate copolymer (EVA) have been used in various adhesive applications; however, such polymers have many problems in their end use applications. For example, semi-crystalline linear low density polyethylene (LLDPE) can be used in hot melt adhesive applications where the crystalline network formed on cooling makes a good adhesive free of tack, but the high level of crystallinity causes the material to be brittle. For this reason other monomers, such as vinyl acetate (VA) or alpha-olefins, are often co-polymerized with ethylene to break up some of the crystallinity and soften the adhesive. Thus, the use of ethylene-based hot melt adhesives with vinyl acetate comonomers is limited when low temperature conditions of use are desired.
PCT Publication Nos. WO 1997/33921 and WO 98/03603 teach the use of ethylene-based copolymers in hot melt adhesives and that these copolymers are useful in some applications, but suffer in that they have higher melt viscosity, hence poorer processing, due to higher chain entanglement density, poorer adhesion to low energy surfaces due to mismatched surface energy, and poorer cohesion due to lower glass transition temperature (Tg) than propylene-based copolymers. Tg values of polyethylene and polypropylene are about −128° C. and −10° C., respectively. Generally, the observed strength or cohesion is governed by polymer chain segmental viscosity. Therefore, to compare the cohesion of two polymers the test temperature and polymer Tg are normalized in order to minimize the contribution from differences in viscoelasticity.
U.S. Pat. No. 5,397,843 discloses blended polymer compositions comprising an admixture of (a) a copolymer of ethylene and an alpha-olefin and (b) an amorphous polypropylene, and/or amorphous polyolefin, or mixtures thereof, which have a crystallinity of less than 20 wt. %.
U.S. Pat. No. 5,548,014 discloses hot melt adhesives comprising a tackifier and a high weight average molecular weight (MW), narrow molecular weight distribution (MWD), narrow CD (composition distribution) ethylene/alpha-olefin copolymer(s) and a low MW, narrow MWD ethylene/alpha-olefin copolymer prepared with either supported or unsupported cyclopentadienyl derivatives of Group IV, and catalysts for applications in hot melt adhesives, particularly in automotive product assembly, packaging and food packaging.
U.S. Pat. Nos. 5,530,054 and 6,207,748 disclose hot melt adhesives comprising ethylene/alpha-olefin copolymers prepared with either supported or unsupported metallocene-alumoxane catalysts for hot melt adhesives applications in automotive product assembly, packaging and food packaging, wherein these ethylene copolymers have MW ranging from about 20,000 to about 100,000, and comonomer weight percent ranging from about 6 to about 30.
PCT Publication No. WO 97/33921 discloses adhesives and processes for preparing the same, comprising at least one first homogeneous ethylene/alpha-olefin interpolymer, and optionally at least one tackifier, and optionally at least one plasticizer.
PCT Publication No. WO 98/03603 discloses hot melt adhesives comprising at least one first homogeneous linear or substantially linear ethylene polymer having a particular density and melt viscosity at 350° F. (177° C.), and an optional wax and tackifier. In particular, disclosed is a hot melt adhesive characterized by: a) at least one first homogeneous linear or substantially linear interpolymer of ethylene with at least one C3 to C20 alpha-olefin interpolymer having a density from 0.850 g/cm3 to 0.895 g/cm3; b) optionally, at least one tackifying resin; and c) optionally, at least one wax wherein the hot melt adhesive has a viscosity of less than about 5000 cPs (50 grams/(cm·sec) or 5000 mPa·sec) at 150° C.
There is a need for improved adhesive compositions comprised of blends of C2-based and propylene-based polymers, and processes or methods for making such compositions, which have important attributes from both types of polymers. Specifically, adhesive compositions comprised of C2-based polymers having higher Brookfield viscosities and lower heat of fusion, and propylene-based polymers having Brookfield viscosities in a narrow range and higher or similar heat of fusion. This invention meets these and other needs.